Methods of making high voltage resistant members

ABSTRACT

The invention discloses resulting products and a method of producing high voltage resistant members employed in electrical installations such as bushings and insulators. The method comprises placing a sheath of unvulcanized elastomer on a core, which for example is a fiberglass rod, for example by extruding the sheath thereon, and mounting vulcanized sheds on the sheath therealong, then vulcanizing the sheath to form an integrated high voltage resistant member. The sheath is unvulcanized when the sheds are mounted thereon and then the sheath is heated to vulcanize it, in order to bond it to the sheds and to the core. The sheds also can be molded directly onto the sheath together therewith in steps along the length of the core. For producing a bushing, the sheath is placed on a core formed of a conductive stud wrapped with paper and metal foil and then impregnated with a suitable hardenable resin. The sheds may be stretched on a mandrel at room temperature and then chilled prior to insertion upon the sheath; subsequent warming shrinks the sheds for a tight grip on the sheath.

CROSS REFERENCE TO EXISTING APPLICATION

This is a continuation-in-part to copending U.S. patent application Ser.No. 19,849 filed Mar. 12, 1979, now U.S. Pat. No. 4,312,123.

TECHNICAL FIELD

This invention relates to methods of producing members of various typesemployed in electrical installations, such as high voltage resistantinsulators for outdoor service in supporting high voltage conductors,and bushings for conducting electricity through a wall of a building ora casing of a transformer or circuit breaker. The invention relateslikewise to high voltage resistant members so produced.

BACKGROUND ART

High voltage resistant members for outdoor service are known, whichtypically are made of a fiberglass reinforced resin bonded rodsurrounded by a suitable weather resistant material of a shape andconstruction which will shed rain. Metal fixtures are attached to therod, for example in service as an insulator, to allow connection to aconductor at one end and to a support structure at the other end.

A number of problems have been found in design, manufacture andapplication of high voltage resistant members. The problems in part arerelated to the nature of the weather resistant material surrounding therod. Epoxy resins were among the earliest materials used as the weatherresistant material and it was found that employment of hydrated aluminain large quantities improved their tracking and erosion resistance. Thisimprovement, due to the hydrated alumina, also occurs with otherpolymers. The epoxy resin formulations which provide the best electricalperformance, however, generally are rigid and subject to cracking at lowtemperatures, especially if the rod is loaded mechanically in tension orcantilever bending.

Various elastomers have been used to improve flexibility of the weatherresistant material. These elastomers have includedethylene-propylene-diene monomer rubber (EPDM), ethylene-propylenemonomer rubber (EPM), butyl rubber, silicone resins, fluorocarbonpolymers and the like. EPM and EPDM are particularly attractive from acost viewpoint. But the elastomers generally are formed by molding underpressure, though a few are castable. The castable elastomers usuallysuffer from inadequacies in this regard, such as poor tear strength, andusually are not able to incorporate sufficiently large quantities ofhydrated alumina to give the needed performance in tracking and erosionresistance. The moldable elastomers require prohibitively large moldsand presses, if the high voltage members are to be molded in one piece.

A number of patents relating generally to construction and manufactureof such high voltage resistant members are listed hereafter:

U.S. Pat. Nos. 1,991,700, Rost; 2,683,185, Morrison; 2,732,423,Morrison; 2,945,912, Imhof; 3,001,004, Black; 3,001,005, Soinnenberg;3,118,968, Moussou; 3,152,392, Coppack et al.; 3,328,515, Vose;3,291,899, Ward et al.; 3,356,791, McGowan; 3,358,076, Rebosio;3,446,741, Hervig et al.; 3,531,580, Foster; 3,544,707, Gamble;3,549,791, Yonkers; 3,626,083, Minter et al.; 3,800,111, Holmstrom;3,898,372, Kalb; 4,217,466, Kuhl; 4,246,696, Bauer et al.

British Pat. Nos. 816,926, Coppack; 902,197, Bannerman; 915,052,Sweetland; 1,066,209, Rebosio; 1,116,197, Rebosio; 1,182,045 Rebosio;1,226,265, British Insulated; 1,292,276, Clabburn et al.

West German No. 28 32 543, Trevisan et al.; 1,189,600 Leeds.

U.S. Pat. No. 3,898,372 discloses a method which circumvents the moldingproblem of EPM by molding each shed separately and then mounting thesheds over a fiberglass rod and filling the space between the sheds androd with a silicone grease. This is a simple expedient allowingindividually molded sheds to be produced at low cost. However, such aconstruction creates numerous potential access points for water invasionfrom the outside to the rod. When the rod becomes wet, it failselectrically. Such wetting could occur during high pressure waterwashing of the member, a practice used by many electric utilities toremove accumulated contamination.

British Pat. No. 1,182,045 seeks to eliminate the numerous joints by useof a preformed elastomeric sleeve. The internal surface of the sleeve istreated in order to render it bondable to the rod and its insidediameter must be sufficiently greater than the outside diameter of therod to permit adhesive to be introduced in order to bond the rod and thesleeve together. Adhesive voids can result from such a cumbersomeprocedure.

U.S. Pat. No. 3,112,357 discloses a conductor with hardenable resinouscompositions, but which are not particularly resistant to tracking anderosion when exposed to weather and contamination. Consequently aporcelain housing has been required for outdoor use. Such a procelainhousing is both heavy and fragile.

U.S. Pat. No. 4,217,466 discloses a composite insulator comprising a rodof nonsaponifiable resin reinforced with glass fibers of low alkalicontent, an intermediate layer of a nonsaponifiable moisture repellantpolymer surrounding the rod, and screens surrounding said intermediatelayer, the screens also being of a moisture repellant nonsaponifiablepolymer and containing a filler. Suitable polymers for the screens arethose containing ether or acetal groups, for example, silicone rubber orethylene-propylene rubber. A preferred polymer for the intermediatelayer is a polyfunctional polyorganodimethylsiloxane. The screens may beprefabricated and pushed on a prefabricated rod containing theintermediate layer, or may be cast molded onto said prefabricated rod.

U.S. Pat. No. 4,246,696 discloses a composite insulator comprising aprefabricated glass fiber rod surface treated with a silane, astrengthened extruded rubber layer, and a plurality of prefabricatedscreens slipped over and bonded to the rubber layer. The strengtheningof the extruded layer is carried out by incorporating pyrogenic-obtainedsilicic acid in the rubber.

West German Offenlegungsschrift No. 28 32 543 discloses an electricinsulator for medium and high voltage comprising a glass fiber stem anda skirted coating of elastic organic material, said material beingcompatible with the stem, for example, an ethylene-propylene elastomerwith favorable antitracking and antierosion characteristics, highelasticity and aging and flame resistance. Preferably, the skirtedcoating is a unitary element of the assembly, but my comprise a tubularsleeve and individual shirts. The bond between stem and skirted coatingis made with a curing mixture of weakly unsaturated olefinic polymers,which polymers are analogous to and compatible with theethylene-propylene coating, and which cure at room temperature.

SUMMARY OF THE INVENTION

An object of the invention is to provide a method of producing highvoltage resistant insulator or conductor members suitable for outdoorservice which method and members avoid the stated deficiencies of theprior art.

In accordance with one aspect of the invention there is provided amethod of producing a high voltage resistant member comprising placing asheath of unvulcanized rubber elastomer on a core, such as a fiberglassreinforced resin bonded rod, mounting one or more vulcanized sheds onsaid sheath, and then heating the assembly to vulcanize the sheath andbond the sheath to the core and to the shed, so as to form an integratedhigh voltage resistant member. It should be noted that at the same timethe sheath can be vulcanized to terminal fittings in this method.

In accordance further with the invention, the sheath is extruded ontothe core.

The heating can be effected in a steam or inert gas autoclave in theabsence of oxygen.

In order to achieve heating in an air oven, the sheds are appliedagainst one another to cover the sheath in its entirety.

In accordance further with the invention, the sheds first are expanded,then cooled (the sheds retaining their expanded forms) and then fittedover the sheath. Subsequent warming of the sheds causes them to shrinkby virtue of memory, and to fit tightly onto the sheath.

In accordance with a further aspect of the invention, thee is provided amethod for producing a high voltage resistant member comprising placinga sheath of elastomer on a core, forming grooves in the sheath, and thenmounting vulcanized sheds in the grooves in the sheath, and thenvulcanizing the sheath to form an integrated high voltage resistantmember.

In yet a further aspect of the invention, the sheds are not separatelymolded and the method comprises successively molding, in stepslengthwise along a bare core, the sheath of elastomer and at least oneshed on the sheath. The sheaths in the successive molding steps arevulcanized together to form a continuous sheath on the core.

In a modification of the above method, a sheath is first placed on thecore and the core and sheath are advanced through the mold stepwise andat least one shed of rubber elastomer is molded on the sheath in eachstep.

The rubber elastomer of the sheath is nonvulcanized when placed on thecore and the molding of the sheds on the sheath in each of the steps iseffected at a temperature to produce vulcanization of the sheath andbonding thereof to the core and the sheds to form an integrated highvoltage resistant member.

The invention contemplates also production of conductive members with aninsulative coating, such as bushings (particularly oil-less bushings)wherein the core of the bushing consists of a conductive stud wrappedwith paper and stress grading means such as metal foil, and impregnatedwith a suitable hardenable resin. Epoxy resins are particularly suitablefor this application. This core can be formed separately and the sheathplaced thereover. Or the conductive stud with the paper and metal foilwrapped thereon can be placed within the sheath and the hardenable epoxyresin can be impregnated therein. In this latter example, curing of thehardenable resin must be accomplished at temperatures below that atwhich the sheath will vulcanize. The sheds then are mounted on thesheath after which the assembly is heated to vulcanize the sheath andbond same to the core and to the sheds to form an integrated highvoltage resistant bushing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a portion of a high voltage resistantmember according to a first embodiment of the invention.

FIG. 2 is a similar section according to a second embodiment of theinvention.

FIG. 3 is a similar section according to a third embodiment of theinvention.

FIG. 4 is a sectional view taken at one end of the embodiment of thehigh voltage resistant member of FIG. 1, the opposite end being similar.

FIG. 5 is a diagrammatic illustration of a method for the formation ofhigh voltage members according to the invention.

FIG. 6 is a diagrammatic showing of a method in which the sheds aredirectly molded onto the sheath of the high voltage members.

FIG. 7 is a sectional view of a high voltage resistant bushing producedin accordance with the method of the invention.

DETAILED DESCRIPTION

FIG. 1 of the drawing shows a portion of a long high voltage resistantinsulator for outdoor service. The insulator comprises a rod 1 ofinsulative material preferably constituted as a fiberglass reinforcedresin bonded rod in which the fibers are bonded together with a suitableresin, such as a polyester or epoxy resin, in conventional fashion.

Mounted on the rod 1 is a sheath 2 made of an elastomer formulated forsuch application. For example, the sheath 2 can be constituted of anethylene-propylene-diene monomer (EPDM) containing hydrated alumina inlarge quantity, preferably sixty percent (60%) or more of theformulation. Cross linking or vulcanization may be accomplished by meansof known agents, such as an organic peroxide or a known sulfurcomposition which is activated by heating the polymer to a temperatureof at least about 140° C. and preferably to about 180° C. The materialpreferably should be nontacky when unvulcanized thus permitting easyhandling.

Other suitable elastomeric materials include ethylene-propylenecopolymer (EPM), silicone elastomers, butyl rubber and fluorocarbonpolymers. Silicone resins have been found to be tacky when unvulcanizedand therefore they are less suitable for use in the present invention.

The sheath 2 (shown in FIGS. 1-4) can be placed on the rod 1 by passingthe rod through the crosshead of a rubber extruder 3 as shown on theleft sides of FIGS. 5 and 6. The operation is similar to that usedcommercially for coating wire and cable. The speed of a rod passingthrough a crosshead depends upon the diameter of the rod, typically a7/8" rod will move at several feet per minute through the crosshead. Thespeed is largely dependent on obtaining a satisfactory continuouscoating as the rod exits from the crosshead through a die 4 whichregulates the thickness of the coating. The coating can be on the orderof a thickness between 1/8 and 1/2". Coatings significantly thinner than1/8" may not provide sufficient weather protection for the rod over longperiods of time, and coatings of thickness greater than 1/2" are lesseconomical and appear to provide no significantly better properties.

The extrusion of the elastomeric compound onto the rod does not causethe material to reach temperatures sufficient to cause it to vulcanize.The extruded material is allowed to cool in air and is handled easily atroom temperature in the case of preferred compositions. Rods of anylength may be coated in this manner and successive lengths can be coatedin a continuous operation by simply abutting the following rod againstits predecessor while feeding it into the crosshead.

Separately molded sheds 5, of the same or similar elastomericformulation, are made by standard molding techniques such ascompression, injection or transfer molding. The sheds 5 are formed witha central hole of a diameter slightly less than that of the rod with thesheath 2 thereon. The sheds are expanded to fit over the sheath and arepositioned along the rod at desired locations. The sheds can be spacedat any distance, but most commonly are two to three inches apart. Asmaller spacing tends to bridge when exposed to heavy rainfall andgreater spacings provide less leakage distance than generally desired inmost applications. Nevertheless the sheds easily may be positioned atany desired spacing, which may be at regular or irregular intervals.This ability to space sheds at any interval is an important advance overmethods which depend upon sheds abutting one another, thereby fixingboth their spacing and the leakage distance per unit length, anunnecessary and undesirable design restriction.

The molding of the sheds results in their vulcanization. The sheds canbe heated subsequently in order to enable them to be expanded on amandrel so that the diameter of the central hole will be larger thanthat of the sheath 2. The sheds are then cooled on the mandrel. Uponremoval from the mandrel while still cool, the sheds maintain theirexpanded shapes and can be fitted over the sheath 2 and subsequentlywhen warmed the sheds shrink to a tight fit on the sheath. The sheds maybe warmed, for example by a hot air gun, after being positioned on thesheath. This warming causes the sheds to return to their original sizesand to grip the sheath. The sheath is not vulcanized by this treatment.This expansion and contraction phenomenon is well known for manyelastomeric compositions.

In a variation of the method of the preceding paragraph, the sheds areexpanded at room temperature by means of a mandrel so that the centralhole of the sheds is slightly larger than the diameter of the rod withthe sheath thereon. The expanded sheds, while still on the mandrel, arethen cooled, for example, by refrigeration means, removed from themandrel and slipped over the sheath. Upon warming to room temprature thesheds shrink to their original size, thereby forming a tight grip withthe sheath.

At an end of the rod (as seen in FIG. 4) there is provided a metalfitting 6 which allows connection of the member. The details ofconstruction of the fitting are immaterial to the invention and, solelyby way of example, the fitting comprises a head 7 formed with an ovaleye 8 for attachment to a support or to a high voltage conductor. Theend of the fitting preferably is formed with a recess 9 into which aportion of the sheath 2 can extend. The recess 9 can be between onequarter of an inch to one inch in length in order to provide an adequatesealing area of the sheath 2 to the metal fitting 6. The sheath 2 isremoved from the rod 1 at its end over a distance of several inches toprovide for adequate gripping of the rod 1 by the metal fitting 6. Thefittings can be bonded to the bare rod 1 by means of adhesive or theycan be held by conventional techniques of swaging of the fitting 6 onthe bare end of the rod 1. If adhesive bonding is used, gel temperaturesmust be such that the sheath is not vulcanized prematurely by anyapplied heat. Generally the length of bare rod which is gripped by thefitting is about four times the diameter of the rod for maximumstrength, whether held by adhesives or by compression fitting. Such agrip length is sufficient to result in rupture of the rod in tensionbefore failure of the attachment means between the rod and fitting.

The assembly of the rod 1, sheath 2, sheds 5 and fittings 6 is thenintroduced into an autoclave 10 as represented schematically in FIG. 5and the assembly is then heated to a vulcanization temperature of 140°C. to 200° C. If an adhesive has been employed between the rod and themetal fitting which required an elevated temperature for curing, thiscuring also will take place during the vulcanization heating step in theautoclave 10.

In forming the assembly as shown in FIG. 1, the autoclave 10 is onecontaining a steam or inert gas atmosphere from which oxygen isexcluded, because oxygen at high vulcanization temperatures woulddiscolor the elastomeric polymer if desired light colored formulationsare employed. Pressure is required if the sheds do not entirely coverthe unvulcanized sheath on the rod, inasmuch as the heat necessary tocause vulcanization will generate gaseous products in the elastomericformulation of the sheath that cause it to bloat and erupt in a mass ofblisters and craters. A pressure on the order of several atmospheres,preferably 100 to 300 psi, is necessary to prevent such problems at thevulcanization temperatures of 140° to 200° C. The time required for thevulcanization depends upon the mass of material and the thickness of thesheath to be vulcanized, because the sheath must be heated entirely tothe desired temperature. Vulcanization generally is achieved in 20 to 60minutes. After vulcanization the resulting insulator high voltageresistant member is removed from the autoclave 10 and is allowed to coolto room temperature.

In a modification which allows vulcanization to take place atatmospheric pressure in an air oven, the sheds are constructed as shownin FIG. 2 at 5' where they are formed with collars 5a which abut oneanother so that the sheath 2 is completely covered. In this way,bloating of the sheath is prevented, because formed gases cannot escapeat the surface of the sheath. These gases generally are formed by theperoxide curing agents contained in the elastomer composition or byother chemical components thereof.

The particular shape and construction of the collar 5a is adjustablewithin wide limits and it is only necessary that the sheds and collarsentirely enclose the sheath all along the length thereof. Such aconstruction also fixes spacing of the sheds.

It has been found that in accordance with the method of the inventionexcellent bonds are formed between the sheds and sheath, between thesheath and the core, and between the sheath and the end fittings. As aconsequence, an integrated high voltage resistance member is produced.The bonds between the shed and sheath and that between the sheath andcore are necessary to prevent electrical failure along these interfaceswhile the bond between the sheath and the end fittings is desirable toprevent penetration of moisture to the core at the fittings.

The elastomeric composition of the sheath and the sheds is such as toallow same to extend readily with the core as it expands and contractsdue to temperature variations or changing mechanical loads attemperatures from -40° C. to 70° C. which represents the range oftemperatures experienced in the intended outdoor high voltage insulatoror bushing applications.

In a modification of the invention, as shown in FIG. 3, a modifiedsheath 2' is provided with grooves 11 for accurate and tight placementof sheds 5. In this modification, the sheath 2' can be vulcanized beforethe sheds have been assembled in place. The sheds 5 are bonded to thesheath by means of adhesive or if they are tightly fitted to the sheath,the joints between the sheath and the sheds can be sealed with waterresistant greases. Sheds 5 can be mounted in place by the expansion andcontraction phenomena as previously noted and the presence of thegrooves 11 facilitates tight connection of the sheds with the sheath.

FIG. 6 illustrates a modification in the method of the invention andtherein it is seen that the rod 1 covered with the sheath 2 as deliveredfrom the extruder 3 is supplied into a mold 12 for stepwise molding ofthe sheds 5 along the length of the sheath 2. After each shed 5 ismolded on the sheath 2, the mold 12 is opened and the rod 1 is advancedto the proper position for the next shed 5. Although the mold 12 in FIG.6 is shown as forming a single shed 5 during each molding operation, itis possible according to the invention to mold a plurality of sheds 5 onthe sheath 2 during each molding step.

The sheath 2 may be unvulcanized when it is introduced into the mold 12and molding will take place at a temperature to produce vulcanization ofthe sheath 2 as well as molding of the shed 5 thereon. Though the sheath2 normally would be unvulcanized, it does not need so to be. As thesheds 5 are formed and vulcanized, they bond themselves to the sheath 2.

In a variation, the sheath 2 and the shed 5 can be molded simultaneouslyon the rod 1, and in such variation, the bare rod 1 will be introducedinto the mold 12.

It has been found that the high voltage resistant members obtained bythe methods of this invention are superior to conventional constructionby virtue of the continuous elastomeric covering of the fiberglass rod.Furthermore it is found that the high voltage resistant members aremanufactured easily at reasonable cost with highly integral interfaces.

In particular, there are no access points to the rod 1 and there is noneed for complex and questionable quality steps of surface treatment andbonding of the interior surface of a preformed tube to the rod and tothe sheds 5 as in the prior art. Moreover the high voltage resistantmembers have superior properties, such as tear resistance (as comparedto cast elastomers) particularly the silicone resins of the prior artand they have far superior flexibility, particularly at low temperatures(as compared to cast epoxy) also as previously employed. In addition theability to space the sheds as desired for any application is ofconsiderable value to the customer in solving differing contaminationproblems, which require different amounts of leakage distance foroptimum performance.

FIG. 7 shows a conductive insulated member and more particularly abushing which is used to conduct electricity through a barrier, such asa wall or container surrounding a transformer or circuit breaker. Suchbushings can be capacitively graded or ungraded electrically. Ungradedbushings generally are used only at the lower distribution voltages withair as the dielectric medium. Capacitively graded bushings are morecompact. They generally include metal foils placed concentrically aboutthe conductor with the number and shape of the foils depending uponvarious parameters of the particular application.

FIG. 7 shows a core 19 comprising a conductive rod or tube 20 with acovering 21 formed by winding paper with metal foil interposed thereinon the conductor and impregnating the paper and foil with a hardenableresin, such as an epoxy resin. The covering 21 can be produced inaccordance with U.S. Pat. No. 2,945,912. The paper and metal foil becomeencapsulated in a voidfree cast epoxy mass that both protects andinsulates the windings. Both filled and unfilled formulations of theimpregnating resin may be used.

In conventional solid core bushings it has been found that a heavyfragile porcelain housing is required for weather protection of the coreof the bushing. Additionally, a layer of oil is required between thecore and the procelain housing, so no gap is present in which partialdischarges and radio interference voltages can occur.

The construction as shown in FIG. 7 provides an oil-less bushing whichis particularly suitable for use with electrical equipment filled withsulfur hexafluoride. Moreover it eliminates the need for a porcelainhousing at the upper end of the bushing as well as need for oil to fillthe gap between procelain and core. The procelain housing has beenrequired previously because the preferred hardenable epoxy resincompositions are not particularly resistant to tracking and erosion whenexposed to weather and contamination.

In the construction as shown in FIG. 7 an unvulcanized sheath 22 ofelastomeric materials, corresponding to the sheath 2 in the previouslydescribed embodiments, is fitted around the paper windings and thehardenable resin is impregnated into the windings with the sheath inplace. The resin is cured in the sheath 22 at a temperature below thatwhich will cause the sheath to vulcanize. Sheds 25 corresponding tosheds 5, as shown in the previous embodiments, are then mounted on thesheath. The sheds have been separately formed and vulcanized. A mountingflange 26 is positioned on the covering 21 and directly abuts the lowerend of sheath 22 or may enclose it partially. The assembly is heated,preferably in an autoclave, to a temperature which producesvulcanization of the sheath, and thereby a weather resistant case formedby the sheds and sheath of relatively little weight will be bondeddirectly to the covering 21 and flange 26. The sheath 22 becomesvulcanization bonded to the covering 21, to the shed 25 and to theflange 26.

Although the metal mounting flange 26 has been shown as directly bondedto the covering 21, it is possible to extend the sheath 22 to the levelof the bottom of the metal flange 26. However, the sheath 22 should notextend below the bottom of the metal flange 26, because this portion ofbushing may be immersed in an oil environment and the rubber compositionof the sheath may not be oil resistant.

In a modification of the method, the covering 21 is hardened on theconductor rod or tube 20 and then the unvulcanized elastomeric sheath 22is slid over the coating and the sheds 25 are then placed onto thesheath. The entire assembly is bonded together by means of theunvulcanized sheath 22. Alternately the hardened core can be passedthrough an extruder crosshead to sheath the rod, in a fashion analogousto the process used for fiberglass reinforced resin bonded rods.

The bushings as shown in FIG. 7 eliminate the need for any oil, yet notat the expense of loss of compact size achieved by grading. Mainly thereis no need for the presence of oil between the core 19 and a separateweather resistant porcelain housing as currently used. The eliminationof oil is of particular advantage in certain applications, notably wheresulfur hexafluoride insulated equipment is employed because any oilwhich is present from a leak in the bushing will foul the sulfurhexafluoride filtering system. Moreover, the bushing of FIG. 7eliminates both the need for checking the presence of oil or gas, suchas by the use of sight glasses or pressure gauges, and the need forvarious other parts, such as springs and gaskets, required to maintaintight seals to confine the oil or gas. These bushings which avoid theneed for porcelain housings can be made rapidly as delivery time for theprocelain usually is the single longest time delay in making bushings tocustomers' orders.

Although the invention has been described in conjunction with specificembodiments, it will become apparent to those skilled in the art thatnumerous modifications and variations can be made, without departingfrom the scope and spirit of the invention. Thus for example, while thesheds have been illustrated as being of conical shape, they could alsobe flat sheds extending perpendicular to the sheath.

The insulators described herein are primarily of a tension type. One canalso make a bushing type construction without a conductive stud, butwith optional insulative reinforcing rods in the paper and resin mass.These constructions are particularly suitable for applications in whichthe forces are primarily compressive or cantilever. These constructionsare the subject matter of copending U.S. patent application Ser. No.933,279 filed Aug. 14, 1978 (now abandoned) wherein Mr. Clark Goshall isthe inventor and which is also assigned to the same assignee as thepresent application. The advantages for horizontal (cantilever) use ofsuch constructions is that a fiberglass reinforced resin bonded rodalone can be made with high quality only up to rather distinct limits ofdiameter, and consequently to distinct limits of cantilever strength.The construction according to the present invention can be made muchthicker in diameter with no particular difficulty.

The invention is not to be taken as limiting except as defined in theclaims which follow.

I claim:
 1. In a method of making a high voltage resistant integratedmember characterized by:extruding a sheath of essentially unvulcanizedelastomer on a core, said extrusion being performed at a temperaturebelow the vulcanizing temperature of the elastomer, the elastomer beingselected from the group consisting of ethylene-propylene-dienecopolymers, ethylene-propylene copolymers, and butyl rubber; mounting atleast one fully vulcanized elastomeric shed on the sheath, the shedbeing fabricated from a material of the same or similar elastomerformulation as used for the sheath, and then heating thecore-shed-sheath assemblage to vulcanize the sheath and bond same to thecore and to the shed; an improvement in the step of mounting before thestep of heating to vulcanize comprising: expanding the sheds at roomtemperature to allow fitting them onto the sheath, cooling the shedswhile maintaining them expanded, placing the cooled expanded sheds onthe sheath covered core, and then allowing the sheds to shrink tightlyonto the sheath by warming them to room temperature.
 2. In a method ofmaking a high voltage resistant integrated member in the form of aninsulated oil-less bushing characterized by:forming a core of aconductive rod with paper windings thereabout, said forming includingthe steps of interposing metal foil within the paper windings andimpregnating the windings with a hardenable epoxy resin, and curing saidresin at a predetermined temperature; extruding a sheath of essentiallyunvulcanized elastomer on said core, said extrusion being performed at atemperature below the vulcanizing temperature of the elastomer, theelastomer being selected from the group consisting ofethylene-propylene-diene copolymers, ethylene-propylene copolymers andbutyl rubber; mounting at least one fully vulcanized elastomeric shed onthe sheath, the shed being fabricated from a material of the same orsimilar elastomer formulation as used for the sheath to provide acore-shed-sheath asemblage, and then heating the core-shed-sheathassemblage to vulcanize the sheath and bond same to the core and to theshed; wherein an improvement in the step of mounting before the step ofheating to vulcanize comprises: expanding the sheds at room temperatureto allow fitting them onto the sheath; cooling the sheds whilemaintaining them expanded; placing the cooled expanded sheds on thesheath covered core; and then allowing the sheds to shrink tightly ontothe sheath by warming them to room temperature.
 3. The method accordingto claims 1 or 2 further characterized in that along with the step ofmounting sheds on the sheath there is an additional step of mounting atleast one metal fitting onto the core, and sealably securing the fittingto the core.
 4. The method according to claim 3 further characterized inthat said step of mounting the metal fitting onto the core is performedprior to said vulcanizing step, whereby sid fitting thereby becomesbonded to the sheath.